This invention relates to a differential voltage to current converter for use in a bipolar integrated circuit. The function of such a converter is to generate an output current proportional to a differential input voltage.
One goal in designing such a circuit is the greatest accuracy in its operation. The prior art in such circuits includes a circuit comprising a current source with an output supplying two parallel current paths, each comprising a resistor, an input transistor and a mirror transistor in series. The resistors have equal resistances; the input transistors have emitters connected to the resistors and bases comprising a differential voltage input; and the mirror transistors are connected in a current mirror arrangement so that the current in one is mirrored in the other.
This prior art circuit generates a differential output current, which is obtained from the junction of the input and mirror transistors on the dependent current side. However, this output current flows through only one of the input transistors and thus unbalances the input transistor emitter currents and thus their base/emitter voltages. The difference in base/emitter voltages on the input transistors produces an error in the operation of the device.
In addition, the circuits of the prior art generally require a comparatively large current through the input transistors; and this loads the input circuit providing the differential voltage input.